In the field of semiconductor manufacturing, as well as other fields, a plasma chamber has various possible uses. For example, plasma-enhanced chemical vapor deposition (CVD) is a process used to deposit thin films on a substrate using a plasma chamber. In high level terms, a radio frequency (RF) power supply is coupled to the plasma chamber to supply power to ignite and sustain a plasma from reacting gases, within the chamber, and from which the deposition occurs on a substrate within the chamber. In order to achieve efficient and accurate power transfer between the RF supply and the plasma load, an impedance-matching network is often used to match the load impedance (including the impedance of the plasma) to the output impedance of the power supply.
The source impedance of the RF power supply may vary by application; however, the industry standard is to have a source impedance of 50 Ohms for most RF generators. Load impedance, on the other hand, may vary widely based on a range of variables including, without limitation, generator frequency, power, chamber pressure, gas composition, plasma ignition, and other variations in the plasma load during processing. As a result, in most applications, the matching network is used to adjust the load impedance such that it remains as close as possible to the source impedance (e.g., 50 Ohms in most, but not all, cases). The match network accounts for these variations in load impedance by varying electrical elements to match the varying load impedance to the generator's output impedance.
Match networks typically contain reactive elements, meaning elements that store energy in electrical and magnetic fields as opposed to resistive elements that dissipate electrical power. The most common reactive elements are capacitors, inductors and coupled inductors but others such as distributed circuits are also used. Match networks can also include lossless elements including transmission lines and transformers. The only resistive elements in a match network are typically associated with losses in non-ideal reactive and lossless components or components that do not take part in the impedance transformation such as components for sensing voltage, current, power or temperature.
Match networks can include a number of variable reactive elements. For instance, vacuum variable capacitors can be used. However, these are bulky and expensive. In the alternative, banks of parallel capacitors having different capacitances, and being added or removed from the parallel circuit via electrical switches are also typical. Often, such capacitor banks use high power PIN diodes (controlled by a transistor) to switch the capacitors in and out of the parallel system.
Accurate impedance matching using a match network generally requires a thorough understanding of the characteristics of the reactive elements within the match network. For example, such characteristics may include changes in the reactance of a variable capacitor when in different switched states. During operation, however, interactions between the electromagnetic fields of the various reactive elements within a given match network can lead to unpredictable and highly variable changes in the characteristics of the reactive elements.
It is with these observations in mind, among others, that aspects of the present disclosure were conceived.